Restructure theme.php to define a class Theme

Restructure theme.php to define a class Theme

For various reasons, it's nicer to have a class for theme-file paths
and such. So, I've rewritten the code for determining the locations of
theme files to be more OOPy.

I changed all the uses of the two functions in the module (theme_file
and theme_path) to use Theme::file and Theme::path respectively.

I've also removed the code in common.php that require's the module;
using a class means we can autoload it instead.